Differential regulation of NFAT and SRF by the B cell receptor via a PLCγ-Ca²⁺-dependent pathway

NFAT and SRF are important in the regulation of proliferation and cytokine production in lymphocytes. NFAT activation by the B cell receptor (BCR) occurs via the PLCγ-Ca²⁺-calcineurin pathway, however how the BCR activates SRF is unclear. We show here that like NFAT, BCR regulation of SRF occurs via an Src-Syk-Tec-PLCγ-Ca²⁺ (Lyn-Syk-Btk-PLCγ-Ca²⁺) pathway. However, SRF responds to lower Ca²⁺ and is less dependent on IP₃R expression than NFAT. Ca²⁺-regulated calcineurin plays a partial role in SRF activation, in combination with diacylglycerol (DAG), while is fully required for NFAT activation. Signals from the DAG effectors protein kinase C, Ras and Rap1, and the downstream MEK-ERK pathway are required for both SRF and NFAT; however, NFAT but not SRF is dependent on JNK signals. Both SRF and NFAT were also dependent on Rac, Rho, CDC42 and actin. Finally, we show that Ca ²⁺ is not required for ERK activation, but instead for its association with nuclear areas of the cell. These data suggest that combinatorial assembly of signaling pathways emanating from the BCR differentially regulate NFAT and SRF, to activate gene expression.